DONALD TRUMP tore into former Republican presidential candidate Mitt Romney tonight, branding him a "loser" who would have "got down on his knees" to secure the billionaire's endorsement for his own White House bid.

The flamboyant tycoon dismissed Romney as a "failure and an irrelevance" during a blistering riposte as the Republican establishment continues to flounder over how to stop the Trump train from steaming home.

Earlier the former Massechussets governor - who was trounced by Barack Obama in the last presidential election - launched a desperate attack on Mr Trump branding him a "phony and a fake".

He became the biggest establishment figure yet to take on the business magnate in a sign of the growing concern amongst party moderates that his campaign is gaining unstoppable momentum.